Investigating Slips and Falls:
The Complex Dynamics Behind Simple Accidents

by Barrett C. Miller

Falls are consistently the leading cause of injury-producing accidents. They account for more than 1 million injuries each year in the United States.

Why are there so many falls? In part, it is because we fail to understand the complexity of same-surface slip-and-fall accidents. We assume that people fall because the floor is slick, because they are clumsy or careless, or because they step on a foreign object. These assumptions lead to investigations that are one-dimensional and to repeated accidents at the same location.

To understand causality and to determine responsibility for an accident, the investigator usually eliminates many possible causes. But people can fall for numerous reasons. Reasons include the interaction of the walking surface with shoes; the environment, along with its distractions; and the physical and mental limitations of the victim.

The Mechanics of Walking
The study of energy, as applied to motion, is called kinesiology. Kinesiology provides us with a model to understand normal and abnormal movement patterns. Any movement, from a golf swing to a child climbing a flight of stairs, can be analyzed.

Most movements involve the expansion and contraction of muscles in relation to joints and bones. People use levers to move their bodies. When we understand the patterns in which these levers work, we can determine the cause of deformities and incapacitates of a person in an accident. We cannot assume, however, that abnormal patterns of movement are responsible for an accident. 

Walking and most other motions of the whole body involve the body's center of mass. This theoretical area, commonly called "the center of gravity" (COG), is the balance point around which a movement operates. While complex measurements are necessary to determine the exact location of the center of gravity, it is estimated that the COG in the average person who is standing is about 55 percent of the distance between the floor and the person's height.

This COG is located in the center of the body, as viewed from the side. The location of the center of gravity affects the way a person walks and falls, and may even affect the severity of a fall.

The COG changes during various activities and postures, and also varies according to the build of the person. In walking, the center of gravity is carried alternately over the right and left foot. The average human walking pattern is called "striding bipedalism" because we stand and walk with two feet in contact with the walking surface.

Each step begins with the weight supported for the trailing foot. We swing the striding foot forward and begin to transfer our weight forward to the heel. Our weight rolls toward the toes as our momentum carries us forward, and we begin to swing the alternate leg forward. As the pace increases, an instance of instability occurs between steps at the moment when our weight is transferred to the leading foot.

Essentially, walking results in successive losses of balance. Clayne Jensen, a motion expert, divides the human stride into three parts: propulsion, swing, and catch and support. (C. Jensen, Applied Kinesiology and Biomechanics (1982).) When speed increases to running pace, only one foot is in contact with the surface at a time.

People walk in distinctive ways. There are so many variations of walking that some contend there is no normal walking pattern. For example, some people nearly skip the heel in the forward step, and obese people often step flat-footed because their center of gravity is carried forward. Each walking pattern causes variations in stride.

These methods of walking are generally thought to be incorrect because they are less efficient, but they should not be considered unsafe unless evidence shows they prevent the individual from maintaining balance. Stride and balance can also be affected by the inner ear, medication, and disorders of the central nervous system.

To stand, walk, or climb without falling, we must maintain our center of mass over and within a base area. When someone is standing erect, the base can be considered the normal footprint. The shape, size, and position of the base changes depending on the pattern of movement and the activity. When walking, we constantly readjust our body segments over our base to maintain stability. The brain, vision, body condition, and the nature of the contact with the surface all contribute to the sensitive balance required to maintain walking stability. If a foot slips or is mispositioned, the center of gravity shifts outside the base area. When this happens, we shift our body parts in an attempt to regain equilibrium. If the center of mass cannot be shifted back over the base area, we fall.

Conscious Control
Walking is a motor skill. Once learned, motor acts like walking are initiated in the cerebral cortex without conscious intention or intervention. Obviously, we do not need to tell our body what joint to move or which muscles to energize. The mind orders a whole action, and the details occur without conscious direction.

A national champion golfer was asked to describe the steps involved in making a shot. She said, "I see the shot, then feel it, and then I do it." Walking is the same.

Ordinarily, we do not consciously recognize changes in the walking surface. As we walk, we scan about 10 to 20 feet ahead of us. Our observations operate below the level of the conscious mind.

Most slip-and-fall accidents are caused by unexpected changes in the walking surface. To become aware of the hazard, the change must impinge upon our consciousness in some way.

If we see a potential hazard, we can usually avoid it. It is possible to walk safely on an icy sidewalk, for example, if we see the hazard and adjust our behavior. It's the unseen hazard that places us at maximum risk.

Individual Rights
When walking in a public place, we have the right to be advised of any hazards we are likely to encounter. If we choose to proceed with full knowledge of a hazard, we assume at least some risk. If the hazard has been knowingly or carelessly disguised, an accident can be expected.

A man visiting a public park broke his leg in a fall as he tried to descend three wooden steps to a lower level. His foot had slipped on a defect in the leading edge of a step. The public parks department investigated the accident and painted the steps with a bright orange paint to call attention to the stairs. unfortunately, the paint covered the defect in the step, and, two weeks later, a woman broke her hip when her foot got caught in the same defect.

Lighting can disguise a defect or hazardous condition. A change in the normal walking environment must be visible to the pedestrian and stand out from background stimuli. glare and too much or too little contrast in the walking environment can reduce the efficiency of the eye.

The walking surface should be evenly illuminated and should have a brightness level of at least 20 foot candles - a basic quantitative unit of light measurement. And the contrast (ratio of dark to light) should be no less than 3-1 and no more than 20-1. The measurement of luminosity and contrast requires a simple photographic light meter calibrated to read in foot candles.

Photographs of the accident scene can help the investigator evaluate the effect of light on the mishap. It is important to photograph the site properly, making sure the photographs represent the accident point from the eight compass positions. These photographs often provide unexpected insights. The camera should be held level and pointed straight ahead, instead of at the floor.

An example of the usefulness of photographs can be shown in the case of a woman who fell while walking through a bank. An examination of the photographs showed what had been missed during previous inspections of the scene: a dip in the floor, where a wall had been removed, which could not be seen from the area of traffic flow used by the victim.

Kinds of Falls
Same-surface falls can be classified into four categories:
1. Trip-and-fall accidents, in which pedestrians encounter a foreign object in their walking path;
2. Stump-and-fall accidents, in which a moving foot encounters an impediment in the walking surface, whether it is a tacky point on the surface or a defect that impedes the foot;
3. Step-and-fall accidents, in which the foot finds an unexpected failure or hole in the walking surface; and
4. Slip-and-fall accidents, in which the interface of the shoe and the floor fails to support the walker's center of gravity over the base area.

The slip and fall is the most common accident. Foot contact is broken, and the individual attempts to right himself or herself. Recovery of equilibrium is reflexive and not under conscious control in most cases. If the pedestrian strikes the surface with a fleshy part of the body, the injuries are likely to be minimal. but if the victim strikes a bony body part, the injuries may be more severe.

Measurement of Surfaces
Years before the invention of modern acrylic finishes and attempts to establish standards for regulation of surface slipperiness, the cleaning industry discovered the need to determine slip-resistance.

The bean-bag test was one of the earliest ways, and it can be traced back to the 1930s. A 10-pound bag of beans was placed on a clean piece of burlap. The weighted burlap was pulled across the floor with a spring scale. The point at which the load began to move was noted, and the test was repeated.

A floor that required six pounds of pull was considered safe. A floor with less than five was slippery, and a floor with a drag of more than seven was considered to be tacky. By today's standards, the bean-bag test is primitive, but it provided a simple, reliable way of comparing results and surfaces. A new product could be tested on a small area, and the safe working procedures could be recorded.

The American Society for Testing and Materials and the National Bureau of Standards have studied the reliability of test devices for measuring the floor surfaces and the effect of cleaning materials on walking safety. We can now predict how a floor-care product will behave when applied according to a manufacturer's recommendations. The new tests are merely revisions of the old bean-bag test.

Floor characteristics are now measured in terms of the static or dynamic coefficient-of-friction(COF). Most common machines measure the coefficient-of-friction of a surface by dragging, pushing, or striking a floor area with a shoe material having a known surface characteristic. This coefficient can be determined by taking the drag necessary to first move the object and dividing it by the weight of the object being pulled. a floor with a COF of .5 or less is considered slippery; a floor with a COF of more than .5 is considered safe. 

For forensic analysis of floor materials, Seigler Pendulum Tester, a horizontal drag slip meter, or another type of calibrated test device is often used. These systems of testing floor surfaces depend on a heel and sole material of known slip characteristics, and the accuracy of the systems has been validated by the National Bureau of Standards and other testing agencies. These simple devices can be very useful. They provide the safety investigator a standard with which to compare a floor surface.

Routine Testing
Modern materials and floor-care products are now considered safe and consistent, so builders and service companies no longer test each batch of a product. But this confidence can lead to problems.

A new shopping center in a tropical city, for example, used glazed ceramic tile in its walkway. A woman slipped on the tile and was hurt. The architect had specified an eight-inch strip of ceramic tile with a decorative pattern, and the builder presented the owner with samples. No one knows if the tile manufacturer had recommended the use of this tile on outside walkways, but it was a poor choice. When it rains, as it often does in this tropical city, the sidewalk becomes hazardous.

The static coefficient-of-friction has been measured at .30 on a wet day. In this case, a small number of tiles could have been tested under conditions similar to those in its expected use. The cost wold have been minimal. but now the tile must be completely removed from the shopping center or temporarily fixed by applying an external finish. The removal will cost thousands of dollars.

Types of Floors
The slip-resistance of floor materials has received considerable attention from safety investigators. There is a large body of knowledge about material characteristics. Each material has benefits and disadvantages, and each must be properly installed and maintained to produce a safe walking surface. 

As a rule, the slip-resistance of a floor material is directly proportional to the number of microscopic points that project from its surface. Concrete, for example, is a relatively safe surface, but if it is improperly cured when poured, it becomes a very slippery surface. 

Asphalt tile and vinyl tiles are relatively safe if the surface treatment chosen is correct. Marble and terrazzo surfaces are inherently slippery and should be treated with a sealer containing a high percentage of solids to increase slip-resistance. Floor tile can be very safe or very dangerous, depending on its factory finish and its maintenance.

Floor Finishes and Cleaning Methods
A custodian's locker is a good place to start an investigation of slip-and-fall accidents. The investigator should determine what products are being used and how they are being combined. Are the mops dirty? Does the staff use the same mop for cleaning and disinfecting? How often is the floor finish removed, and what is used to remove it?

A floor finish is a product that is put on top of the raw floor material to protect it, to beautify it, or to change its surface characteristics. A floor finish can be used to make an unsafe floor less slippery or can inadvertently be used to make a safe floor dangerous. Finishes can be either synthetic or organic.

One potentially dangerous floor finish is a paste produce called "carnuba." Carnuba, made from the palm tree, gives the floor a brilliant shine and is preferred by some floor-care companies because of its ease of use, shine, and cost. When used according to manufacturer's recommendations, the product is safe and offers and acceptable standard of floor care.

If problems arise, it is usually because the produce is used improperly. The wax material in carnuba is soft and "walks," that is, it gravitates out of the pedestrian path. The custodian often replaces the thin areas with fresh paste, leaving a heavy build-up toward the outside of the traffic area. This build-up produces a dangerous surface.

Acrylic finishes are also common. Some companies offer special formulations for problem floor surfaces like terrazzo and marble. One manufacturer makes an acrylic finish that contains aluminum oxide flakes. This finish can be applied in restaurants and other locations where floors are usually wet. These acrylics form a one-piece floor surface through interlocking crystallization when the material dries. Acrylics make safe floor surfaces, but are labor intensive and have to be removed when the product builds up. 

Improper cleaning methods can complicate matters further. Oil mops treated with petroleum base sprays can turn a safe floor into a hazardous one. Pine-tar disinfectants used on ceramic floors sometimes leave a slippery residue and cause many bathroom falls.

The National Safety Council reports that may slip accidents are caused by improper cleaning methods and recommends that floors be cleaned only with clean water. If soap or commercial strippers are used on a floor, care must be taken that no residue remains when the floor dries.

A final finish is sometimes applied to the floor by the pedestrian, who can pick up fertilizer and other chemicals on shoes and bring dangerous adherents into a building.

Individuals at Risk
If a victim has been previously injured, is disabled in some other way, or is elderly, we might assume that the physical infirmity caused the accident. But our assumptions can prevent our seeing the actual cause. There is no correct way to stand or walk. What we view as correct posture and movement is often a matter of esthetics. Handicapped people sometimes need to use a high level of conscious intervention in the walking process.

Mrs. Jones is an example. She suffered from polio as a child, and her leg muscles and hip were seriously atrophied. She used leg braces and a cane when she walked. For 35 years, she had walked on ice and snow and climbed stairs and moved about in all kinds of situations. She walked monopedially, that is, she often had only one foot in contact with the floor when she took a step. At the moment when she shifted her weight from one foot to the other, she used the cane for stability.

Mrs. Jones entered a fast food restaurant that displayed a sign indicating it was equipped for handicapped use. It was a bright day, and the restaurant was using subdued lighting for effect. The windows had been treated to cut down light transmission. When entering this environment, it takes between three to five seconds for one's eyes to adjust to the light. For Mrs. Jones, the light at floor level was less than 20 foot candle, and the contrast between the brown ceramic tile and a spilled soft drink was too low for her eyes to measure. Her right foot slipped to the right, and she fell. A close examination showed that the pores of the normally safe unglazed ceramic tile were packed with soap residue. When wet, the spot produced a dangerous condition, and the subdued lighting hid the condition.

Investigators of falls should understand the principles of human movement and have a working knowledge of floor materials, cleaning methods, and lighting.

When reconstructing a slip-and-fall accident, investigators should avoid coming to any conclusion too quickly. The victim should be extensively interviewed, and the verbal report used to focus the investigation. The slip-resistance of the floor should be measured, and accurate measurements of the light intensity and contrast should be made.

In every case, the findings should be compared to building codes and industry standards. Although they are seemingly the simplest of all accidents, falls are anything but.